The Seasons

The seasons are caused by the tilt of Earth's axis (23.4°) and not by the fact that Earth's orbit around the Sun is an ellipse. The average distance of Earth from the Sun is 93 million miles; the difference between aphelion (farthest away from the Sun) and perihelion (closest to the Sun) is 3 million miles, so that perihelion is about 91.4 million miles from the Sun. Earth goes through the perihelion point a few days after New Year's Day, just when the Northern Hemisphere has winter. Aphelion is passed during the first days of July. This by itself shows that the distance from the Sun is not important within these limits. What is important is that when Earth passes through perihelion, the northern end of Earth's axis happens to tilt away from the Sun, so that the areas beyond the Tropic of Cancer receive only slanting rays from a Sun low in the sky.

The tilt of Earth's axis is responsible for four lines you find on every globe. When, say, the North Pole is tilted away from the Sun as much as possible, the farthest points in the North that can still be reached by the Sun's rays are 23.5° from the pole. This is the Arctic Circle. The Antarctic Circle is the corresponding limit 23.4° from the South Pole; the Sun's rays cannot reach beyond this point when we have midsummer in the North.

When the Sun is vertically above the equator, the day is of equal length all over Earth. This happens twice a year, and these are the “equinoxes” in March and in September. After having been over the equator in March, the Sun will seem to move northward. The northernmost point where the Sun can be straight overhead is 23.4° north of the equator. This is the Tropic of Cancer; the Sun can never be vertically overhead to the north of this line. Similarly the Sun cannot be vertically overhead to the south of a line 23.4° south of the equator—the Tropic of Capricorn.

This explains the climatic zones. In the belt (the Greek word zone means “belt”) between the Tropic of Cancer and the Tropic of Capricorn, the Sun can be straight overhead; this is the tropical zone. The two zones where the Sun cannot be overhead but will be above the horizon every day of the year are the two temperate zones; the two areas where the Sun will not rise at all for varying lengths of time are the two polar areas, Arctic and Antarctic.